The crystal structure and electrical properties of the oxide ion conductor Ba3WNbO8.5

Abstract

The structural and electrical properties of the hexagonal perovskite derivative Ba₃WNbO_8.5 have been investigated. Ba₃WNbO_8.5 crystallises in a hybrid of the 9R hexagonal perovskite and palmierite structure as recently reported for the novel oxide ion conductor Ba₃MoNbO_8.5. Ba₃WNbO_8.5 is also an oxide ion conductor and appears to exhibit oxide ionic conduction with negligible electronic conductivity over a wider pO₂ range than Ba₃MoNbO_8.5. A neutron diffraction study has revealed that at 20 °C the average structure of Ba₃WNbO_8.5 contains just 13% of W(1)/Nb(1)O₄ tetrahedra within the average structure of Ba₃WNbO_8.5 in comparison to 50% of Mo(1)/Nb(1)O₄ tetrahedra in Ba₃MoNbO_8.5. The presence of (M/Nb)O₄ tetrahedra with non-bridging apical oxygen atoms is an important prerequisite for the ionic conduction observed in the Ba₃MNbO_8.5 system (M = W, Mo). The strong reduction in the ratio of (M/Nb)O₄ tetrahedra to (M/Nb)O₆ octahedra upon replacement of W⁶⁺ for Mo⁶⁺ results in a reduction in the ionic conductivity by an order of magnitude at 450 °C. The bulk conductivities converge upon heating so that at 600 °C the bulk conductivity of Ba₃WNbO_8.5, σ_b = 0.0017 S cm⁻¹, is comparable to that of Ba₃MoNbO_8.5 (σ_b = 0.0022 S cm⁻¹). The results demonstrate that other members of the Ba₃MM′O_8.5 family can support oxide ion conductivity and further studies of hexagonal perovskite derivatives are warranted.